This application is based on Japanese Patent Application 2000-84762, filed on Mar. 24, 2000, the entire contents of which are incorporated herein by reference.
a) Field of the Invention
The present invention relates to a wavelength converting device, and more particularly to a wavelength converting device for converting a wavelength by utilizing four-wave mixing. A wavelength converting device is used, for example, in wavelength multiplexing optical communications. Since light waves before and after wavelength conversion have a phase conjugation relation, a wavelength converting device is used for compensating for refractive index dispersion of an optical fiber uses for long distance optical transmission.
b) Description of the Related Art
When a pump wave having an angular frequency of xcfx89p and a signal light wave having an angular frequency of xcfx89s are input to an active layer whose carrier distribution is in an inversion state, a light wave having an angular frequency of (2xcfx89pxe2x88x92xcfx89s) is output because of non-degenerate four-wave mixing. It has been proposed to utilize this phenomenon for wavelength conversion from a light wave of an angular frequency of xcfx89s into a light wave of an angular frequency of (2xcfx89pxe2x88x92xcfx89s).
Semiconductor laser devices and semiconductor optical amplifier devices generally use bulk semiconductors and multi quantum well structures. In wavelength conversion in an active layer of these semiconductor devices through non-degenerate four-wave mixing, as a detuning angular frequency (xcfx89pxe2x88x92xcfx89s) becomes large, a conversion efficiency lowers. If the detuning angular frequency is negative, the conversion efficiency is low.
It is an object of the present invention to provide a wavelength converting device and a wavelength converting method capable of suppressing the conversion efficiency from being lowered at a large detuning angular and obtaining a relatively high conversion efficiency even at a negative detuning angular frequency.
According to one aspect of the present invention, there is provided a wavelength converting device, comprising: a multi quantum well layer having input and output planes and a lamination of first and second quantum well layers each sandwiched between barrier layers and defining a quantum well, a band gap of the second quantum well layer being larger than a band gap of the first quantum well layer; a pair of electrodes to be used for flowing current through the multi quantum well layer along a thickness direction thereof; a first optical system for inputting a signal light wave to the multi quantum well layer via the input plane of the multi quantum well layer; and a second optical system for propagating an output light wave output from the output plane of the multi quantum well layer.
As current is flowed through the multi quantum well layer, the multi quantum well layer may enter the state that the first quantum well layer functions as a gain region and the second quantum well layer functions as an absorption layer, respectively relative to the light having a certain wavelength. The third order non-linear susceptibility by the carrier density pulsation effect in the gain region is proportional to a product of a gain and a differential refractive index. The third order non-linear susceptibility by the carrier density pulsation effect in an absorption region is proportional to a product of an absorption and a differential refractive index, and its sign is opposite to the sign of the third order non-linear susceptibility in the gain region. The third order non-linear susceptibility in a multi gain region is a sum of the third order non-linear susceptibilities in both the regions. If the values of the third order non-linear susceptibilities in both the regions are set to have values nearer to each other, the third order non-linear susceptibility by the carrier density pulsation effect can be made small. The third order non-linear susceptibility by the spectral hole burning effect is proportional to a difference between the gain and absorption, and does not depend on the differential refractive index. Accordingly, even if the third order non-linear susceptibility by the carrier density pulsation effect is made small, the third order non-linear susceptibility by the spectral hole burning effect can be maintained large. Since the third order non-linear susceptibility by the spectral hole burning effect is less dependent to the detuning angular frequency, the detuning angular frequency dependency of the wavelength conversion efficiency can be reduced.
According to another aspect of the present invention, there is provided a wavelength converting device, comprising: a multi quantum well layer having input and output planes and a lamination of first and second quantum well layers each sandwiched between barrier layers and defining a quantum well, a band gap of the second quantum well layer being larger than a band gap of the first quantum well layer; and a power source for flowing current through the multi quantum well layer in a thickness direction thereof so that a quasi Fermi level difference in the multi quantum well layer becomes larger than the band gap of the first quantum well layer and smaller than the band gap of the second quantum well layer.
An inversion distribution state of carrier densities occurs in the first quantum well layer. The inversion distribution state does not occur in the second quantum well layer. In this case, the first quantum well layer functions as the gain region and the second quantum well layer functions as the absorption layer, respectively relative to the light having a certain wavelength. Therefore, the third order non-linear susceptibility can be controlled in the manner described above.
According to another aspect of the present invention, there is provided a wavelength converting device, comprising: a multi quantum well layer having input and output planes and a lamination of first and second quantum well layers each sandwiched between barrier layers and defining a quantum well, a band gap of the second quantum well layer being larger than a band gap of the first quantum well layer, the second quantum well layer containing compressive strain and the first quantum well layer not containing strain; and a pair of electrodes to be used for flowing current through the multi quantum well layer in a thickness direction thereof.
The quantum well layer having compressive strain has a differential refractive index larger than that of the quantum well layer having no strain. Therefore, under the conditions that the third order non-linear susceptibility of the multi quantum well layer by the carrier density pulsation effect is constant, it becomes possible to raise the gain of the first quantum well layer and lower the absorption of the second quantum well layer. The third order non-linear susceptibility by the spectral hole burning effect can therefore be made large.
According to another aspect of the present invention, there is provided a wavelength converting method comprising the steps of: preparing a wavelength converting device having a multi quantum well layer having input and output planes and a lamination of first and second quantum well layers each sandwiched between barrier layers and defining a quantum well, a band gap of the second quantum well layer being larger than a band gap of the first quantum well layer; flowing current through the multi quantum well layer so that a quasi Fermi level difference in the multi quantum well layer becomes larger than the band gap of the first quantum well layer and smaller than the band gap of the second quantum well layer; inputting a signal light wave to the quantum well layer via the input plane; inputting a pump wave to or generating a pump wave in the multi quantum well layer, the pump wave having such a wavelength that the pump wave is amplified in the first quantum well layer and attenuated in the second quantum well layer; and picking up a wavelength converted output light wave from the output plane of the multi quantum well layer.
As the current having the above-described magnitude is flowed through the multi quantum well layer, the third order non-linear susceptibility by the spectral hole burning effect can be made large as described above. The detuning angular frequency dependency of the wavelength conversion efficiency can therefore be made small.
As described above, the detuning angular frequency dependency of the third order non-linear susceptibility of a multi quantum layer can be made small by using quantum well layers having different band gaps. It is therefore possible to reduce the wavelength dependency of the efficiency of wavelength conversion using non-degenerate four-wave mixing.